1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly to a semiconductor memory device having ferroelectric capacitors.
2. Description of the Related Art
In recent years, the technology of nonvolatile memory which is electrically erasable and programmable has advanced very much. Thanks to this technology advance, various types of nonvolatile memories have been developed. Among these nonvolatile memories is a ferroelectric memory which has ferroelectric capacitors made of ferroelectric material.
Each of the ferroelectric capacitors is polarized, and stores data or does not store data in accordance with the direction in which it is polarized, as is disclosed in Published Unexamined Japanese Patent Application No. 63-201998 corresponding to U.S. patent application Ser. No. 013,746 (filed Feb. 12, 1987). The operating principle of the ferroelectric capacitor will be explained briefly.
The ferroelectric capacitor comprises a plate made of ferroelectric material and two electrodes formed on the opposing two surfaces of the plate. The ferro-electric material has a polarization-voltage characteristic which exhibits hysteresis. Hence, once polarized by applying a voltage to it in one direction or the opposite direction, the ferroelectric capacitor remains polarized (or charged) even after the application of the voltage is stopped. This phenomenon is known as "residual polarization" or "residual charging".
When a voltage is applied to the polarized ferroelectric capacitor so that a voltage larger than a coercive electric field is applied to the ferroelectric capacitor, the ferroelectric capacitor is polarized in an opposite direction.
When to the ferroelectric capacitor is applied a voltage having the same polarity as that of the polarized ferroelectric capacitor, there is accumulated in the ferroelectric capacitor an electric charge which is no more than the value corresponding to the capacitance of the ferroelectric capacitor, just in the same way as in ordinary capacitors made of dielectric material. In contrast, when a voltage is applied to the ferroelectric capacitor at an opposite polarity to the polarity of the polarized capacitor, thus applying an opposite electric field more intense than a predetermined value to the ferroelectric capacitor, the plate is polarized in the opposite direction.
When the ferroelectric capacitor is polarized in the opposite direction, it flows a far greater electric charge than when it is polarized in the same direction as the pre-polarized direction. Whether the plate is polarized in one (first) direction or the other (second) direction can be determined by detecting the charge in the plate in terms of, for example, a voltage drop. Hence, the ferroelectric capacitor can be said to store a "0" when polarized in the first direction and to store a "1" bit when polarized in the second direction. Alternatively, it can be said to store a "0" when polarized in the second direction and to store a "1" bit when polar zed in the first direction.
As has been pointed out, the charge, whether large or small, remains in the plate even when a voltage is no more applied to the plate. The ferroelectric memory is, therefore, nonvolatile. The ferroelectric memory operates at high speed. More specifically, both its write speed and its read speed are about tens of bits per nanosecond (several 10 bit/nsec). Great demand for a ferroelectric memory, i.e., a nonvolatile memory comprising ferroelectric capacitors is now made, and this ferroelectric memory will be used in increasing numbers in the very near future.
When a voltage is applied to the plate of a ferroelectric capacitor in the first direction in order to read the data from the capacitor, the ferroelectric capacitor is not reversely polarized if the capacitor has been polarized in the first direction and stores a "1" bit. However, when a voltage is applied to the ferroelectric capacitor of a ferroelectric capacitor in the first direction in order to read the data from the capacitor, the ferroelectric capacitor is reversely polarized if the capacitor has been polarized in the second direction and stores a "0" bit. In this case, the plate must be polarized in the second direction after the "0" bit has been read from the ferroelectric capacitor. In the case where a voltage is applied to the capacitor in the second direction, thus reading a "1" bit a ferroelectric capacitor, the plate of this capacitor must be polarized in the first direction after the "1" bit has been read from the capacitor.
Obviously, the ferroelectric memory is basically a destructive-reading type. Each ferroelectric capacitor is reversely polarized very frequently.
As is known in the art, the polarization characteristic of a ferroelectric capacitor gradually deteriorates, or the capacitor gradually "wears out," as it is reversely polarized repeatedly. In other words, the more often the plate is polarized reversely, the lower its polarization characteristic. It is generally said that the ferroelectric plate has its polarization characteristic critically deteriorated when it has been reversely polarized 10.sup.12 times or more. The frequency of reverse polarization of the ferroelectric capacitor is one of the most prominent determinants of the lifetime of the ferroelectric capacitor. Hence, it is desirable that the plate be polarized as seldom as possible in order to prolong the lifetime of the ferroelectric capacitor.
Published Unexamined Japanese Patent Application No. 64-66899 corresponding to U.S. patent application Ser. No. 069,390 (filed Jul. 2, 1987) discloses the technique of incorporating a ferroelectric memory in a ordinary semiconductor memory, so that data which need to be preserved can be stored in the ferroelectric memory which is nonvolatile. This technique helps to reduce the frequency of reversely polarizing the capacitor incorporated in the ferroelectric memory. Since the ferroelectric capacitor is not reversely polarized so often, it does not wear out fast. The ferroelectric memory can therefore have a long lifetime.
However, the ordinary semiconductor memory is nothing more than one provided with a back-up memory. If the ordinary semiconductor memory and the ferroelectric memory are formed on the same substrate, as in most cases, the resultant composite memory is rather large and has complicated circuitry. The composite memory cannot easily be manufactured in the form of an integrated circuit.
It is true that the ferroelectric memory has a great prospective use. However, its lifetime is far less long than is desired, due to the wearing-out of the ferroelectric capacitors.